As a developing process of a developing apparatus with a nonmagnetic one component developing system in recent years, it has been known a contact developing system in which a developing roll to which a toner (developer) has been attached is uniformly contacted to a photoreceptor by pressure to carry out the development (see FIG. 3).
In general, an electrostatic latent image is formed onto a surface of a photoreceptor drum based on predetermined image information by the means of the electrostatic latent image, while a toner supply roller supplies the toner onto a surface of a developing roll, a developer amount regulation member (hereinafter also referred to as “regulation blade”.) regulates the toner supplied on the developing roll to a uniform thickness, and a uniform thin layer of the toner is formed on the surface of the developing roll.
The toner is successively adhered by the developing roll on the surface of which is formed the toner thin layer to the electrostatic latent image formed on the photoreceptor drum at a nip portion or a proximity part of the photoreceptor drum. Toner development is thus carried out.
For obtaining a uniform and high density image in such a contact development system, it is important to form a toner attached layer which is a uniform and thin layer on the surface of the developing roll. Therefore, it is necessary to sufficiently retain a conveying amount of the toner by forming a uniform rough surface at the surface of the developing roll, and to secure uniform gaps with the regulation blade.
The developing roll generally has a structure in which a rubbery elastomer having conductivity is accumulated at a columnar surface of a core material including a metal, a fibrous material or a resin molded material. And by providing a coating layer including a resin to the surface of the rubbery elastomer, it has been taking measures against (1) the problem that a component contained in the rubber is transferred to the surface, so that a mating member is contaminated, (2) the problem that a volume resistance value is markedly changed by the change in an environment, (3) the problem that coefficient of dynamic friction is large, so that a large torque is required for rotation in relation to the friction with the contacting mating member, (4) the problem of tackiness of the toner, tackiness to a mating member, occurrence of frictional sound, and stain adhesion, etc., due to the characteristics of high tackiness characteristics and high friction coefficient.
In Patent Document 1, it has been disclosed a method of forming a uniform rough surface by dispersing resin particles for the purpose of roughening the surface onto the surface of the developing roll for making the surface of the developing roll a uniform rough surface.
For forming unevenness onto the surface of the developing roll by particles, it is necessary to add many particles, and when hard resin particles occupy almost all the part of the surface of the developing roll, a coating layer itself is hard and projected parts of the coating layer are also hard so that deterioration of the toner proceeds.
When the toner layer is formed, the projected parts of the surface of the developing roll are rubbed particularly by a blade, so that a frictional heat is generated by rubbing whereby it is easily melted. When the toner is melted, there is a problem that filming in which the toner is adhered to the surface of the developing roll easily occurs. The toner is likely melted by deterioration and the problem is easily generated.
Accompanying with high speed and high resolution of an image forming apparatus in recent years, the toner is made low melting point and small diameter, whereby it is more likely melting by heat.
A toner conveying amount can be made large in accordance with the degree of roughness of the surface of the developing roll, so that, in Patent Document 2, it has been disclosed a means to make the particles to be formulated in the surface layer large for the purpose of making roughness of the roll surface large.
However, if the surface roughness becomes too large, roughness of the resulting image or image unevenness is generated, or fogging property is lowered, and it is difficult to form uniform rough surface over the whole part of the roll surface.
If there is fluctuation in roughness of the roll surface, there are fear that the toner conveying amount is deviated and the toner charging amount becomes non-uniform. Further, the surface of the spherical particles is hard since it is a resin, and depending on a kind of the toner, there is a possibility of giving damage to the toner and of generating filming.
In Patent Document 3, Patent Document 4 and Patent Document 5, there are disclosed a means of lowering a surface hardness of recessed parts at which much toner is retained than that of projected parts formed at the surface whereby toner stress is reduced.
However, according to these means, hard particles are contained in the coating layer, so that toner stress is particularly remarkable, whereby sufficient printing durability cannot be obtained.
In Patent Document 6, there is disclosed a technique using a silicone rubber binder as a binder, and polymethyl-silsesquioxane fine particles as resin particles.
In this case, the binder is soft, but the polymethyl-silsesquioxane fine particles are resin particles which are hard and they deteriorate the toner.
Also, the binder is a silicone rubber binder so that adhesiveness with the rubber substrate layer is poor, and filming or tackiness of the toner is generated in some cases.
Further, there is a problem that these resin particles are precipitated during the process of coating them onto a roll surface, so that it is difficult to coat them so as to uniformly exist onto the roll surface.
In Patent Document 7, there is disclosed a coating layer in which particles which comprise a material having low compression set, for example, a silicone resin or a urethane resin is uniformly dispersed in a binder. As the coating layer, a polyurethane resin has been preferably used as a material which can maintain stable friction charge.
However, for projecting small particles to the surface of the coating layer, a thickness of the coating layer must be thin.
Accordingly, in general, it was necessary to make the coating layer thin to cause surface roughness by making the particles small. However, when the coating layer is made thin, there is a problem that the effect of the rubber which is the substrate cannot sufficiently be avoided as mentioned above.